1. Field of the Invention
The present invention relates generally to heat exchangers, and more particularly to a new and improved device for shot-peening the inside surface of the U-bend region of oval-shaped heat exchanger tubes employed, for example, within a steam generator heat exchanger of a pressurized water nuclear reactor (PWR) facility so as to impart to the inside surface material of each heat exchanger tube U-bend region compressive stresses for counteracting residual tensile stresses developed within the U-bend regions of each heat exchanger tube as a result of the cold-bending fabrication processing of the heat exchanger tubes, whereby stress corrosion cracking failures of the tubes are effectively eliminated or at least substantially reduced.
2. Description of the Prior Art
A nuclear reactor produces heat as a result of the fission of nuclear material which is disposed within fuel rods, the fuel rods being secured together so as to define fuel assemblies. The fuel assemblies define the nuclear reactor core, and the cores is disposed within a reactor or pressure vessel. In commerical nuclear reactor facilities, the heat produced by means of the fission processes is utilized to generate electricity. In particular, conventional facilities usually comprise one or more primary core coolant flow loops and heat transfer or exchange loops, and may also comprise a corresponding number of secondary flow and heat transfer or exchange loops to which conventional steam generators and steam turbines, as well as electrical generators, are fluidically and mechanically connected, respectively. a typical energy conversion process for such commercial nuclear reactor facilities would therefore comprise the transfer of heat from the nuclear reactor core to the primary coolant flow and loop system, from the primary coolant flow and loop system to the secondary coolant flow and loop system by means of suitable heat exchangers, and finally from the secondary coolant flow and loop system to the steam generators by means of further suitable heat exchangers. The generated steam is then of course transmitted ot the steam turbines to which the electrical generators are operatively connected, and from which the electricity is ultimately generated.
In a pressurized water reactor (PWR), water serves as the reactor core coolant, and there is no provision of a secondary coolant flow and loop system as is required in connection with a liquid metal-cooled fast breeder reactor (LMFBR) wherein the secondary coolant flow and loop system, in which there is disposed a liquid metal, such as, for example, sodium, as in the case of liquid sodium also being disposed within the primary coolant flow and loop system, serves as a buffer zone between the primary core liquid metal coolant flow and loop system and the water-steam generator system. In a pressurzied water reactor, therefore, the water is circulated only through the primary coolant flow and loop system which may typically comprise the nuclear reactor core, a heat exchanger, and a circulating pump. Some nuclear reactors may have more than one primary coolant flow loop within the primary coolant flow and loop system, and in this instance, the nuclear reactor core and the reactor pressure vessel, within which the core is disposed, are connected in common to each of the primary coolant flow loops. The heat generated by means of the nuclear core is thus removed therefrom by means of the reactor core coolant which is conducted into the reactor pressure vessel and through the reactor core. The heated reactor core coolant then exits from the nuclear reactor core and the reactor pressure vessel so as to flow through the heat exchangers which serve to transfer the heat from the heated nuclear reactor core coolant water to the water being conducted through the heat exchangers whereby steam is generated for use within the steam turbines. The steam turbines are then of course utilized to drive the electrical generators for generating electrictiy. the cooled reactor core water coolant disposed within the primary flow and loop system is then recirculated back to the ractor pressure vessel and the nuclear reactor core by means of the primary flow loop system circulating pump, and the coolant cycle is repeated.
Within one type of conventional, exemplary heat exchanger system defined between the primary nuclear reactor core water coolant flow and loop system and the water-steam generator flow path through which water is conducted for the generation of steam, the heat exchanger vessel has a lower portion thereof divided into two separate and isolated sections for the introduction and withdrawal, respectively, of hot nuclear reactor core coolant water. The two lower sections, in effect, define headers into which the core coolant water is introduced and from which the coolant water is withdrawn, and the upper boundary of the headers is defined by means of a horizontally disposed tubesheet to which the ends of numerous reactor core coolant water conduction tubes or conduits are fixedly secured. In particular, the reactor core coolant water conduction tubes or conduits have oval-shaped configurations with the tubes only defining one-half of a closed oval path. The open ends of the tubes are thus secured within the tubesheet such that the tubes themselves extend vertically upwardly from the tubesheet, and consequently, one end of each tube is fluidically connected to the reactor core coolant water inlet header for the reception of the reactor core coolant water, while the other end of each tube is fluidically connected to the reactor core coolant water outlet header for the discharge of the cooled reactor core coolant water to be recirculated back to the reactor core. Water, from which steam is to be generated, is introduced into the heat exchanger at a level which is above the tubesheet, and generated steam is discharged from the heat exchanger from the uppermost portion thereof as a result of the heat exchange process occurring within the heat exchanger.
The nuclear reactor core coolant water conduction tubes or conduits are fabricated from a suitable metal alloy material, such as, for example, INCONEL 600, in view of the fact that such material exhibits desirable operational characteristics which are critically requisite for service within heat exchanger systems. For example, such material exhibits good heat transfer properties, corrosion resistance, and good structural integrity under high-temperature conditions. In addition, such material is readily formable and weldable, and, of course, is commercially available. As noted hereinabove, the water conduction tubes have a substantially inverted U-shaped configuration, and the tubes are fabricated by means of well-known cold bending techniques. Unfortunately, as a result of such fabrication techniques, residual tensile stresses are developed within the U-shaped tubes or conduits, particularly upon the inside surfaces thereof at the relatively sharp bend locations. Such stresses have been shown to cause stress corrosion cracking failures within the heat exchangers which have conventionally necessitated the plugging or closing of the ends of the cracked or failed coolant water conduction tubes so as to effectively remove such failed or cracked coolant water conduction tubes from the heat exchange service. Subsequently, of course, such failed or cracked conduction tubes will have to be repaired or replaced, and therefore, it can readily be appreciated that such prematurely terminated service lives of the core coolant water conduction tubes result in substantially enhanced operating costs for the power plant facilities in view of increased costs for replacement per se of the cracked or failed conduction tubes, maintenance personnel time for accomplishing the repair or replacement of the failed or cracked core coolant water conduction tubes, and lost revenues to the powerplant facility as a result of those time periods during which electrically generated power production is terminated, curtailed, or reduced while the necessary repairs or replacement operations for such failed or cracked core coolant water conduction tubes within the particular heat exchanger system are being performed.
It has been determined, and confirmed by microhardness measurements and stress corrosion cracking tests, that if compressive surface stresses, which would effectively counteract the aforenoted deleterious effects of the residual tensile stresses, could be provided upon the interior surfaces of the reactor core coolant water conduction tubes within the U-bend regions thereof, then the stress corrosion cracking failures exhibited within the conduction tubes can be eliminated or substantially reduced.
Accordingly, it is an object of the present invention to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof.
Another object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to effectively overcome the deleterious effects of the residual tensile stresses developed within the U-bend regions of conventional U-shaped heat exchanger tubes.
Yet another object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to overcome the various operational disadvantages or drawbacks characteristic of conventional U-shaped heat exchanger tubes as noted hereinabove.
Still another object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof wherein the heat exchanger tubes are employed within nuclear reactor heat exchange systems.
Yet still another object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to counteract the residual tensile stresses developed within the U-bend regions of the U-shaped heat exchanger tubes as a result of the cold bending fabrication thereof.
Still yet another object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to prevent stress corrosion cracking of the U-shaped heat exchanger tubes.
A further object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to prevent stress corrosion cracking failures within the U-shaped heat exchanger tubes whereby the service lives thereof are substantially increased.
A yet further object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to substantially eliminate or reduce stress corrosion cracking of the U-shaped heat exchanger tubes within the U-bend regions thereof as a result of residual tensile stresses originally developed within the U-bend regions of the U-shaped heat exchanger tubes as a result of the cold bending fabrications techniques employed in connection therewith.
A still further object of the present invention is to provide new and improved apparatus for imparting compressive stresses to the inside surfaces of substantially U-shaped heat exchanger tubes within the U-bend regions thereof so as to substantially eleminate or reduce stress corrosion cracking failures of the U-shaped heat exchanger tubes within the U-bend regions thereof and thereby effectively reduce maintenance and repair costs in connection with the heat exchanger system.